1. Field of the Invention
The present invention generally relates to a filter element having a piezoelectric thin-film resonator and a filter device including the filter element, and more particularly, to a duplexer and a high-frequency circuit that are required to have high power durability to be used at the front end of a radio terminal.
2. Description of the Related Art
In recent years, wireless terminals such as portable telephones have been rapidly becoming smaller and lighter. In this trend, components for those terminals are expected to become smaller and exhibit higher performance. In duplexers to be used at the front ends of RF circuits, for example, conventional dielectric filters are being replaced with surface acoustic wave (SAW) filters that are much smaller than the conventional dielectric filters.
SAW filters, however, have a drawback in inherently having low power durability, since minute comb-like electrode patterns are used in those SAW filters. So as to increase the power durability of SAW filters, improvements have been made on the electrode materials and designs.
Such SAW filters to be employed in duplexers often include a ladder-type circuit. Hereinafter, those filters having ladder-type circuit structures will be referred to as “ladder filters”. In the following, the structure of a ladder filter will be described, with reference to the accompanying drawings.
As shown in FIG. 1, a “base section” of a ladder-type circuit includes a single-terminal pair resonator (a single-terminal pair SAW resonator in a SAW filter, hereinafter referred to simply as a “resonator”) in each of a series arm and a parallel arm. In the following description, a resonator arranged in a series arm will be referred to as a “series-arm resonator S”, and a resonator arranged in a parallel arm will be referred to as a “parallel-arm resonator P”.
A ladder filter has a structure in which two or more “base sections” are connected while image impedance matching is performed between the sections. Hereinafter, each “base section” will be referred to as “one stage”. As an example of such a structure, FIG. 2 shows an equivalent circuit diagram of a ladder filter 100 having a four-stage structure. In practice, any two neighboring resonators (series-arm resonators S2 and S3, parallel-arm resonators P1 and P2, and parallel-arm resonators P3 and P4 in FIG. 2) may be combined into one, so as to obtain desired characteristics or to reduce the filter size. FIG. 3 shows an example of a structure in which every two neighboring resonators of the ladder-type circuit shown in FIG. 2 are combined into one. In FIG. 3, a series-arm resonator S23 is formed by combining the series-arm resonators S2 and S3 of FIG. 2, a parallel-arm resonator P12 is formed by combining the parallel-arm resonators P1 and P2 of FIG. 2, and a parallel-arm resonator P34 is formed by combining the parallel-arm resonators P3 and P4 of FIG. 2. Here, so as not to change the impedance in the equivalent circuit before and after the combining of the resonators, the capacitance of the combined series-arm resonator S23 is halved, and the capacitance of each of the combined parallel-arm resonators P12 and P34 is doubled.
Japanese Unexamined Patent Publication No. 10-303698 (hereinafter referred to as “Prior Art 1”) discloses an example improvement in designs to increase the power durability of a multi-stage ladder filter. As shown in FIG. 4, a ladder filter 200 in accordance with the disclosure of Prior Art 1 has two series-arm resonators S1a and S1b connected in series, instead of the single series-arm resonator S1 located at the first stage when seen from the signal input side in FIG. 3.
Although improvements such as the above have been made to obtain higher power durability, currently available ladder filters do not exhibit power durability that is high enough for practical use.
Meanwhile, more and more attention has been drawn to filters using piezoelectric thin-film resonators (the piezoelectric thin-film resonators being hereinafter sometimes referred to simply as “resonators”, and the filters with the piezoelectric thin-film resonators being hereinafter sometimes referred to as “piezoelectric thin-film filters”), because those piezoelectric thin-film filters have higher power durability and exhibit better filter characteristics than SAW filters. A piezoelectric thin-film resonator normally includes a substrate and a laminated resonator formed on the substrate. The laminated resonator includes a piezoelectric thin-film and a pair of electrode films that sandwich the piezoelectric thin-film. The laminated resonator having such a structure needs to be acoustically insulated from the substrate. Therefore, there is a cavity formed immediately under the laminated resonator, or there is an acoustic multi-layer film formed by laminating alternately two types of films having different acoustic impedances but both having a thickness of ¼ wavelength.
When an AC voltage is applied to the pair of electrodes in the above structure, the piezoelectric thin-film sandwiched by the electrodes vibrates in the thickness longitudinal direction due to a reverse piezoelectric effect. As a result, the piezoelectric thin-film resonator exhibits electric resonance characteristics. This proves that a single-terminal pair piezoelectric thin-film resonator can be represented by the same equivalent circuit as that of a single-terminal pair SAW resonator, and electrically behaves in the same manner as a single-terminal pair SAW resonator. It is therefore possible to employ a single-terminal pair piezoelectric thin-film resonator, instead of a single-terminal pair SAW resonator, in the above ladder filter.
For the above reasons, a piezoelectric thin-film filter has higher power durability than a SAW filter, but needs to have even higher power durability to be put into practical use. Japanese Unexamined Patent Publication No. 2002-198777 (Prior Art 2) discloses an improvement in the power durability of a ladder filter using a single-terminal pair piezoelectric thin-film resonator in the same manner as that disclosed in Prior Art 1. More particularly, Prior Art 2 discloses a ladder filter using a single-terminal pair piezoelectric thin-film resonator that is formed by connecting two or more resonators in series, instead of using a single series-arm resonator. As an example of such a ladder filter, FIG. 5 shows an equivalent circuit of a structure in which a series-arm resonator on the signal input side is divided into two resonators connected in series. FIG. 6 is a plan view of a ladder filter 300 having the above structure. In FIGS. 1 through 6, like resonators (all the series-arm resonators and parallel-arm resonators) of SAW filters and piezoelectric thin-film filters are denoted by like reference numerals.
When two or more resonators connected-in series are employed instead of a single series-arm resonator, however, the number of resonators increases accordingly, and the area of the divided resonators is larger than the area of the single series-arm resonator, resulting in a larger necessary total area. If a single resonator is divided into n (n being a positive integer), the area of the divided resonators is n times larger than the area of the single resonator, so that the impedance of the divided resonators becomes the same as the impedance of the single resonator. Also, to increase power durability as much as possible, not only the series-arm resonator on the signal input side but also all the other series-arm resonators are often divided. In such a case, the total necessary area of the resonators becomes very large, and hinders decrease of the device size. In a case of a membrane-type piezoelectric thin-film resonator in which a cavity for acoustic insulation from the substrate is formed under each resonator, the piezoelectric thin-film resonator breaks easily as the area of the resonators becomes larger, resulting in a difficult problem of having a poorer yield.